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Hekia Parata or no Hekia Parata in the job, Cabinet ministers had few illusions from the start about the difficulty of persuading Christchurch's citizens of the desirability of post-earthquake school closures whoever was Minister of Education.

National's nervousness about the closures triggering a much wider political backlash in the city against the governing party was plain in yesterday's partial backdown from the initial proposals announced in such messy fashion last September.

The number of school closures and "mergers" has been reduced from 31 to 19 - which is about the annual average across New Zealand in recent years.

In one stroke, the Cabinet has cut the number of affected pupils by about half from just over 7300 to 3800 out of an overall student population in the greater city of around 72,000.

But the strongest pointer to how National has been feeling the heat is the decision that the majority of the region's schools still earmarked for closure will shut their doors at the end of the final term this year rather than in 2015 or later as had initially been mooted.

Ministers are clearly punting that if affected pupils are in their new school at the beginning of 2014 then they (or more importantly vote-wise their parents) will have adjusted by the time the election rolls around later in the year.

Weighing on John Key and his colleagues' minds is the need to buttress National's hold on the the city which applauded the party for its handling of the post-quake recovery by increasing National's party vote in Christchurch at the last election from just under 90,000 to just over 95,000.

With Labour's share in spectacular decline - from around 97,000 in 2005 to just over 51,000 at the last election - National won the party vote in the supposed Labour fortresses of Christchurch Central and Christchurch East.

To hold onto power after the next general election, National's strategists somehow have to stop those cross-over voters who have no real allegiance to their party shifting back to Labour.

The school rationalisation exercise was always going to jeopardise National's retention of that "soft" vote. Much of the damage from the quakes was in east Christchurch.